Polymer composite materials include a matrix material and one or more reinforcing materials. The reinforcing materials enhance the strength and/or stiffness of the product while the polymer matrix allows the composite to be formed in complex shapes. The earliest composite materials were reinforced with paper or cotton fabric. Later polymer composites were reinforced by chopped or broken reinforcing fibers randomly disturbed within the matrix. The discontinuous nature of the reinforcing fibers prevented them from achieving significant reinforcement because the strength between isolated fibers was limited by the strength of the matrix material itself or by the adhesion between the matrix material and the fibers.
Later polymer composites incorporated woven mats of reinforcing fibers. The woven mats allowed the full strength and stiffness of the fibers to be employed when the composite was loaded in tension, and to a lesser extent when loaded in compression and flexion. Woven mats also allowed the strands of the reinforcing fibers to be oriented in the optimal direction, or a combination of directions, depending on the expected loading of the part. However, composites manufactured from woven mats greatly increased manufacturing cost and complexity because the mats generally need to be pre-cut and laid in the mold by hand.
3D printing is the process of creating a finished part by depositing (adding) layer after layer of material, until the desired part is obtained. The oldest form of 3D printing is stereolithography, where a bath of liquid polymer is selectively polymerized at its surface by UV radiation. The solidified polymer is then submerged into the liquid so that additional layers can be built upon the previous layers.
The next major innovation in 3D printing was the development of filament deposition modeling (FDM). FDM melts a polymeric or metallic filament and deposits the molten material in thin layers. FDM is faster than stereolithography and allows a wider variety of printing materials. However, neither FDM printing or stereolithography is well suited to fabricating structural components. The first 3D printing system suitable for the fabrication of structural components was selective laser sintering. In this system, a thin layer of metal powder is deposited onto the print surface and selectively sintered by a laser. Another layer of powder is applied over the previously sintered layer, and the process is repeated, so that the second layer is sintered to the first. While these sintered metal components fabricated are suitable for structural applications, their relatively high density makes them unsuitable for some application. Various attempts have been made to adapt FDM printing to create structural composites by adding reinforcement fibers into the polymer filament. While this does improve the mechanical properties, the strength of the fabricated components remains insufficient for use in high-performance applications.
Automated tape placement is a highly precise process for creating composites from unidirectional pre-impregnated textiles (prepreg) textiles. It can be thought of as a more advanced version of the hand lay-up FRP processes, where the human operator is replaced by a robot. This permits very precise fiber placement, but it can only be used on relatively simple curvilinear surfaces. Composites fabricated by automated tape placement are cured by the same methods as hand lay-up.